Electric double layer capacitors, which have features of long cycle life and a wide working temperature range, are attracting attention as a new type of electric storage device that can replace secondary batteries.
A power supply system will be described in which electric double layer capacitors (hereinafter also referred to simply as “capacitors”) are used as an electric storage device, with reference to FIG. 23.
Direct current power supplied from a direct current power source 1 such as a solar cell is temporarily stored in an electric storage unit 21 implemented with a plurality of capacitors in an electric storage apparatus 2. Unlike secondary batteries, with capacitors, the inter-terminal voltage varies significantly in proportion to the amount of accumulated charge, and thus the power stored in the electric storage unit 21 cannot be supplied directly to a load. For this reason, the power stored in the electric storage unit 21 is supplied to a power conversion device 3 such as a DC-DC converter or a DC-AC inverter to stabilize voltage, and thereafter supplied to a load 4.
In the case where the load 4 is driven with direct current power, a DC-DC converter or the like is used as the power conversion device 3. A control unit 22 controls charge and discharge of the electric storage unit 21.
In the power supply system shown in FIG. 23, the power conversion device 3 has an acceptable input voltage range. Accordingly, in order to continuously supply stable power to the load, it is necessary to maintain the output voltage of the electric storage unit 21 (hereinafter referred to as the “electric storage unit voltage”) Vt within the acceptable input voltage range of the power conversion device 3.
Since a single capacitor has a low output voltage, the electric storage unit 21 is often implemented with a plurality of capacitors that are connected in series. It is also often the case that a plurality of capacitors are connected in parallel, in order to secure the required amount of accumulated charge. Accordingly, the electric storage unit 21 including capacitors in the electric storage apparatus 2 are usually configured such that a plurality of capacitors are connected in series and in parallel.
In order to improve charge/discharge characteristics and the depth of discharge, such a conventional electric storage apparatus 2, in which the electric storage unit 21 is implemented with a plurality of capacitors, uses a charge control method and discharge control method using a combination of two methods, namely, “series/parallel switching control” and “parallel monitor”. Hereinafter, an electric storage apparatus 2 that uses the two methods disclosed in Patent Document 1 will be described with reference to the drawings.
First, series/parallel switching control will be described. Series/parallel switching control is a method for improving charge/discharge characteristics and the depth of discharge used in an electric storage apparatus 2 in which the electric storage unit 21 is implemented with a plurality of capacitors that are connected in series or in parallel.
The electric storage unit 21 of Patent Document 1 that uses series/parallel switching control is configured as shown in FIG. 24. Specifically, a single circuit block (hereinafter referred to simply as a “block”) includes a pair of capacitors C, C having an equal electrostatic capacity, and a plurality of switches S that switch the pair of capacitors C, C between series connection and parallel connection, and blocks thus configured are connected in series in n stages (B1 to Bn).
Next, a series/parallel switching control method will be described, taking as an example a case where the electric storage unit 21 is configured with blocks (B1 to B3) in three stages as shown in FIG. 25. FIG. 26 is a diagram showing only the connection states of capacitors C obtained by omitting illustration of the switches of the electric storage unit 21 shown in FIG. 25.
In the case where charging is started from the state in which all capacitors C that constitute the electric storage unit 21 shown in FIG. 25 are in a fully discharged state, switches S12, S22 and S32 shown in FIG. 25 are first turned on (i.e., closed), and switches S11, S13, S21, S23, S31 and S33 are turned off (i.e., opened), whereby as shown in FIG. 26A, the capacitors C of all blocks are connected in series, and charging is started in this state.
As charging progresses, charges are stored in each capacitor C and the electric storage unit voltage Vt rises. Each time the electric storage unit voltage Vt reaches an upper limit value of the acceptable input voltage range of the power conversion device 3, the switches (S11 to S33) shown in FIG. 25 are turned on or off as appropriate, whereby charging is performed such that the capacitors C of each block are switched in sequence from a series connection to a parallel connection in a predetermined order, the order being, for example, FIG. 26B→FIG. 26C→FIG. 26D, until the capacitors C of all blocks are connected in parallel, so that the electric storage unit voltage Vt falls within the acceptable input voltage range of the power conversion device 3.
At the time of discharging, each time the electric storage unit voltage Vt reaches a lower limit value of the acceptable input voltage range of the power conversion device 3 as the electric storage unit voltage Vt declines, the switches (S11 to S13) shown in FIG. 25 are turned on or off as appropriate, whereby discharging is performed such that the capacitors C of each block are switched in sequence from a parallel connection to a series connection in a predetermined order that is the reverse of the order used at the time of charging, such as the order of FIG. 26D→FIG. 26C→FIG. 26B→FIG. 26A. As described above, the series/parallel switching control improves charge/discharge characteristics and the depth of discharge by maintaining the electric storage unit voltage Vt within the acceptable input voltage range of the power conversion device 3.
FIG. 27 is a schematic diagram showing a temporal transition of the electric storage unit voltage Vt during the charging process and discharging process described above. The symbols (a) to (d) shown in the bottom of FIG. 27 indicate time periods corresponding to the connection states of the capacitors C in the electric storage unit 21 shown in FIG. 26A to 26D.
Next, “parallel monitor” will be described. Generally, the electrostatic capacity of capacitors varies significantly from capacitor to capacitor. Accordingly, when a plurality of capacitors connected in series are charged, the capacitors become fully charged from those having a smaller electrostatic capacity. If charging is continued, the capacitors having a smaller electrostatic capacity are overcharged, causing a degradation, or in the worst case, a breakage.
To address this, in an electric storage apparatus in which capacitors are used in the electric storage device, it is often the case that a circuit called a “parallel monitor” implemented with a resistor R and a switch S as shown in FIG. 28A is provided between each pair of capacitors C. When the inter-terminal voltage of each capacitor C exceeds an upper rated voltage (an upper limit value of the inter-terminal voltage at which the capacitor can be used safely), the parallel monitor turns on the switch S as shown in FIG. 28B so as to forcibly bypass charge current Ic, thereby preventing overcharge of the capacitor C.
Next, problems encountered with the charge control method for an electric storage apparatus of Patent Document 1 will be described. Even if the electric storage unit is implemented with a plurality of capacitors having the same nominal electrostatic capacity, the series/parallel switching control reduces the amount of charge that flows into the capacitors C of the blocks that are connected in parallel to approximately half the amount of charge that flows into the capacitors C of the blocks that are connected in series. Furthermore, actual capacitors have electrostatic capacity errors and differences in self discharge characteristics, and thus the inter-terminal voltage varies from capacitor to capacitor.
Due to such variations, when any one of the capacitors C of the blocks connected in series reaches the upper rated voltage (i.e., been fully charged), in order to prevent overcharge, the capacitor C needs to, with the use of the parallel monitor, maintain the inter-terminal voltage at a level less than or equal to the upper rated voltage until the capacitors C of the other blocks connected in parallel are fully charged. As a result, heat loss occurs due to the resistance of the parallel monitor, reducing charge efficiency.
That is, with the conventional charge control method disclosed in Patent Document 1, there are very large variations in the inter-terminal voltage of the capacitors between the blocks, and the parallel monitor requires a long operation time to prevent overcharge, and thus heat loss increases and as a result charge efficiency is reduced.
In order to solve the above-described problems, the inventors have developed a series/parallel switching control method that suppresses variations in the inter-terminal voltage of all capacitors that constitute an electric storage unit (see Patent Document 2).
With the charge control method for an electric storage apparatus developed by the inventors, the parallel monitor is used not only to prevent overcharge, but also to perform correction (called “normal correction”) such that the inter-terminal voltage of each capacitor is within a predetermined range by controlling the parallel monitor at a fixed interval, as well as to prevent so-called “cross current” that occurs when the capacitors of each block are switched from a series connection to a parallel connection. With a combined use of the charge control method for an electric storage apparatus with a series/parallel switching control method, which will be described later, it is possible to reduce the operation time of the parallel monitor and heat generated by the operation of the parallel monitor, as compared to those of the method of Patent Document 1, and charge efficiency can be enhanced.
The method for controlling series/parallel switching of blocks disclosed in Patent Document 2 will be described taking an example a case where the method is applied to an electric storage unit 21 implemented with three blocks (B1, B2, B3) shown in FIG. 29. FIG. 30 is a diagram showing, in a simplified form, the connection states of capacitors C obtained by omitting illustration of the switches of the electric storage unit shown in FIG. 29.
In a charging process, when all capacitors C are in a fully discharged state, switches S13, S01, S23, S02 and S33 are turned on, and switches S11, S12, S21, S22, S31 and S32 are turned off, whereby a state as shown in FIG. 30A in which all capacitors C are connected in series is obtained, and charging is started from that state.
Then, at the time when the electric storage unit voltage Vt reaches the upper limit value of the acceptable input voltage range of the power conversion device 3, the switches (S01 to S33) are controlled as appropriate, whereby the capacitors C of any one of the three blocks (B1 to B3) of the electric storage unit 21 are switched to a parallel connection, so as to reduce the electric storage unit voltage Vt to be within the acceptable input voltage range of the power conversion device 3.
At this time, a block having the highest sum (hereinafter referred to as the “block voltage”) of the inter-terminal voltages of two capacitors C of a block is preferentially connected in parallel. Specifically, the connection state is switched from the state shown in FIG. 30A to one of the states shown in FIGS. 30B, 30C and 30D, and charging is continued.
Then, during the time until the electric storage unit voltage Vt again reaches the upper limit value of the acceptable input voltage range of the power conversion device 3, the inter-terminal voltages of all capacitors are measured at a fixed interval, a single block having the highest block voltage is selected, the capacitors of the block are connected in parallel, and the capacitors of the other blocks are connected back in series, and this operation is repeated. Specifically, an operation of switching, for example, the connection state from the state shown in FIG. 30B to one of the states shown in FIGS. 30C and 30D is repeated.
Furthermore, charging is continued, and when the electric storage unit voltage Vt again reaches the upper limit value of the acceptable input voltage range of the power conversion device 3, in order to reduce the electric storage unit voltage Vt to be within the acceptable input voltage range of the power conversion device 3, the number of blocks that are connected in parallel is increased to 2, and charging is continued. Specifically, the connection state is switched from one of the states shown in FIGS. 30B, 30C and 30D to one of the states shown in FIGS. 30E, 30F and 30G, and charging is continued. At this time as well, two blocks having the highest and second highest block voltages are selected, and the capacitors of the selected blocks are connected in parallel.
Then, during the time until the electric storage unit voltage Vt again reaches the upper limit value of the acceptable input voltage range of the power conversion device 3, the inter-terminal voltages of all capacitors are measured at a fixed interval, two blocks having the highest and second highest block voltages are selected, the capacitors of the selected blocks are connected in parallel, and the capacitors of the other blocks are connected in series. Specifically, an operation of switching, for example, the connection state from the state shown in FIG. 30E to the state shown in FIG. 30F or 30G is repeated.
When charging further progresses, and the electric storage unit voltage Vt again reaches the upper limit value of the acceptable input voltage range of the power conversion device 3, in order to reduce the electric storage unit voltage Vt to be within the acceptable input voltage range of the power conversion device 3, the connection state is switched to the state shown in FIG. 30H so that the number of blocks connected in parallel is three, and charging is continued until the inter-terminal voltage of any one of the capacitors reaches the rated voltage.
The electric storage unit voltage Vt is set so as to be within the acceptable input voltage range of the power conversion device 3 when the capacitors C of all blocks (B1 to B3) are connected in parallel as shown in FIG. 30H and all of the capacitors C are substantially fully charged.
On the other hand, in a discharging process, when all capacitors C are in a fully charged state, discharging is started from the state shown in FIG. 30H (the state in which all blocks are connected in parallel), and when the electric storage unit voltage Vt is reduced to the lower limit value of the acceptable input voltage range of the power conversion device 3, the capacitors C of any one of the blocks are switched to a series connection. This increases the electric storage unit voltage Vt, and discharging is maintained such that the electric storage unit voltage Vt is within the acceptable input voltage range of the power conversion device 3. At this time, blocks having higher block voltages are preferentially connected in series in sequence. Specifically, the connection state is switched from the state shown in FIG. 30H to one of the states shown in FIGS. 30E, 30F and 30G, and discharging is continued.
Then, discharging further progresses, and during the time until the electric storage unit voltage Vt again reaches the lower limit value of the acceptable input voltage range of the power conversion device 3, an operation of measuring the inter-terminal voltages of all capacitors at a fixed interval, selecting a single block having the highest block voltage, connecting the capacitors of the selected block in series, and connecting the capacitors of the other blocks in parallel is repeated, and discharging is continued. Specifically, the operation of switching the connection state from, for example, the state shown in FIG. 30E to the state shown in FIG. 30F or 30G is repeated, and discharging is continued.
When discharging further progresses, and the electric storage unit voltage Vt again reaches the lower limit value of the acceptable input voltage range of the power conversion device 3, in order to increase the electric storage unit voltage Vt to be within the acceptable input voltage range of the power conversion device 3, the number of blocks that are connected in series is increased to 2, and discharging is continued. Specifically, the connection state is switched from one of the states shown in FIG. 30E, 30F and 30G to one of the states shown in FIG. 30B, 30C and 30D, and charging is continued. At this time as well, two blocks having the highest and second highest block voltages are selected, and the capacitors of the selected blocks are connected in series, and discharging is continued.
Then, discharging further progresses, and during the time until the electric storage unit voltage Vt again reaches the lower limit value of the acceptable input voltage range of the power conversion device 3, an operation of measuring the inter-terminal voltages of all capacitors at a fixed interval, selecting two blocks having the highest and second highest block voltages, connecting the capacitors of the selected blocks in series, and connecting the capacitors of the other blocks in parallel is repeated, and discharging is continued. Specifically, the operation of switching the connection state from, for example, the state shown in FIG. 30B to the state shown in FIG. 30C or 30D is repeated, and discharging is continued.
When discharging further progresses, and the electric storage unit voltage Vt again reaches the lower limit value of the acceptable input voltage range of the power conversion device 3, in order to increase the electric storage unit voltage Vt to be within the acceptable input voltage range of the power conversion device 3, the connection state is switched to the state shown in FIG. 30A in which all capacitors are connected in series, and discharging is continued.
In the manner described above, discharging is performed such that the electric storage unit voltage Vt is maintained within the acceptable input voltage range of the power conversion device 3 by switching the connection state of the blocks between series connection and parallel connection.
As described above, a feature of the series/parallel switching control method of Patent Document 2 is that, unlike the method of Patent Document 1, the order of the blocks that undergo series/parallel switching and the pattern of series/parallel switching are not fixed, and series/parallel switching is performed such that variations in the inter-terminal voltage of the capacitors are suppressed at a fixed interval. By doing so, the inter-terminal voltage of the capacitors C of the blocks can be finely controlled, and furthermore, variations in the inter-terminal voltage of all capacitors C that constitute the electric storage unit 21 can be suppressed.